1. Field of the Invention
The present invention relates to a solid-state semiconductor device having excellent high-speed transfer characteristics.
2. Description of the Prior Art
Conventionally, various line sensors and image sensors, as shown in FIG. 1A, using a charge transfer device, such as a CCD, BBD or the like, have been considered. In such sensors, high integration and high transfer speed are being advanced.
On the other hand, in sensors, where an improvement in the aperture ratio is desired, a structure, such that the area of the light shielding portion in the photosensitive surface is made to be as small as possible, is needed. In addition, in many cases, a transparent electrode such as polysilicon or the like is used as a transfer electrode in order to significantly increase the light incidence into the photoelectric converting section.
However, since such poly-silicon has a relatively low conductivity and it is also necessary to improve the aperture ratio as mentioned above, it is more thinly formed.
Therefore, a distributed parameter circuit as shown in FIG. 1B is equivalently formed by resistors which the transfer electrode itself has and capacitors between the semiconductor substrate and the electrode.
FIG. 1A is a diagram showing an example of a pattern of a transfer electrode PE. Also, R denotes resistors and C indicates capacitors. Thus, as shown in FIG. 2, the control waveform at an input terminal a is gradually smoothed as shown at b and c as it is transmitted, causing various problems.
Namely, since the control waveform is smoothed at the portion near the central portion of the transfer electrode pattern, the potential well under the electrode is made shallow, causing the transfer efficiency to deteriorate. When a high voltage is applied to the input terminal a for prevention of such deterioration of the transfer efficiency, the power loss becomes extremely large.
On the other hand, since the shapes of the potential wells which are respectively formed at the central and peripheral portions of the transfer electrode pattern differ, the shading phenomenon is significant. In particular, such a problem becomes larger as the frequency of the control signal, which is applied to the transfer electrode, becomes higher.
Generally, in the case of realizing a high resolution of the image sensor, the number of bits which are read out in a unit time has to be increased. However, such a drawback becomes a large obstacle for making the transfer speed high.